Choosing the Right Trench Cover to Halt Migration of Fines

Through the years, best practices have changed for protecting aggregate

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An installer asked: Do you know of any studies comparing use of straw/hay, building paper and geotextile fabrics in terms of their ability to prevent fine solid migration into a sewage treatment trench? At the date of this writing, we have not found a reference that provides any direct comparison of these common materials when doing the final cover over sewage treatment trenches or beds. Perhaps one of our readers can point us to such a study done locally through a university, government or private entity.

Even though we have been somewhat stumped, we thought it would be a good idea to write about the final cover over sewage treatment trenches; what our goals are and how we can accomplish them or potentially cause some problems.

Everyone should be aware of the general profile of sewage treatment trench, whether it is fed by gravity or pressure distribution. The trench is excavated 1 to 4 feet wide (anything wider is defined as a bed); the most common width is 3 feet, but the excavation width is usually determined by the bucket size used on the backhoe unless there is a specific specification for a specific application. 

Some type of media is placed in the trench. The function of the media is to maintain the structure of the trench; in some cases provide potential storage space for effluent; and protect the infiltrative surface of the soil for acceptance and treatment. There is distribution piping; typically 4-inch-diameter PVC sewage pipe for gravity or 1- to 2-inch-diameter for pressure distribution. The piping is covered and protected by the media. A cover material is placed over the trench before backfilling, followed by soil and topsoil backfill to establish vegetation.

The right rock

The question from the installer related specifically to cover over an aggregate, or as we would refer to a rock-filled trench. Over the past 50 years, cover material and the type of media available for us in trenches has changed. We have moved from only or primarily aggregate to chambers, gravelless pipe, PVC pipe wrapped in Styrofoam peanuts to numerous other materials. They can be used as long as they fulfill the functions of maintaining trench stability, storage area and protection.

The rock would be ¾- to 2 ½-inch in size so it can be easily worked, leveled off and moved around in the trench. The rock needs to be of durable material; a rock that would break down or slake when water is added would not be suitable. An example of unsuitable material would be limestone rock. As water is added, the rock dissolves, moves to the bottom of the trench and effectively seals it off.

The question relates directly to the cover provided over the top of the trench media. The primary purpose of this cover material is to prevent soil particles associated with the trench backfill from filtering into the aggregate or other media. This would potentially move downward to the soil surface, effectively clogging both the media and the soil we are depending on to accept the effluent.

It is always interesting to go back to some of the early design manuals to see what they have to say about different aspects of installation procedures and note how things have changed over time. 

Here is a quote from a 1981 Minnesota Design manual: “Over the top of the rock place a 4-6-inch layer of marsh hay or straw, and then a layer of untreated building paper or newspapers, or other permeable material.” Wow! Who reading this would like to spread out newspapers as the cover material today? Some who had read this may have actually done it, but we are sure no one wants to go back to those days, and it was only 40 years ago. Jim was doing education sessions at that time advocating such a practice; Dave was still about a decade away and things had already changed.

If you go back a little further in design manuals to the early ’70s or ’60s, there is no mention of building paper as a cover addition. The building paper was added because there were observations that straw or hay was not always doing an adequate job of keeping fine soil particles from migrating downward. 

The right fabrics

The introduction of geotextile fabrics eliminated a messy and time-consuming activity of installation by replacing straw and building paper. All very good except there was a learning curve for installers relative to the fabrics. They are not all created for the same purposes. Some of them used in road construction are purposely weaved to prevent moisture movement. We do not want to use these types of fabrics with our systems. 

We want to take advantage of any positives associated with evapotranspiration from our trenches. Our permeable fabric should allow passage of moisture both ways while eliminating migration of the soil fines.

A final note, we have seen numerous systems over the last few decades where straw was used and there was no evidence of soil migrating into the trenches. Conversely, we have seen trenches that have been totally sealed off; so is that a product of the straw itself or was it just bad practice on the part of the installer? Maybe some of both; but use of geotextiles has simplified trench installation. 


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